1. Field of the Invention
The present invention relates generally to gas sensors and, more specifically, to a method of operating a calorimetric gas sensor for improved accuracy and detection limit.
2. Description of the Related Art
Calorimetric gas sensors are potentially useful for measuring on-board the concentration of combustibles in automotive exhaust systems and for correlating increased levels of combustibles with deterioration of the hydrocarbon (HC) efficiency of an automotive three-way-catalyst (TWC). An example of such a calorimetric gas sensor is disclosed in U.S. Pat. No. 5,451,371 to Zanini-Fisher et al. This patent discloses a silicon-based microcalorimetric gas sensor. The detection limit of the microcalorimetric gas sensor was found under laboratory conditions to be sufficiently high to be able to measure hydrocarbon concentration levels in exhaust gas of newer automotive vehicles. However, when these sensors are directly mounted in the automotive exhaust system, the detection limit is reduced. First, the sensor noise increases in a high-velocity, turbulent flow because the effect of local thermal fluctuations, that are not compensated by the differential nature of the sensor, becomes more important. Secondly, the intrinsic zero-offset of the sensor output which derives from small differences in resistance between a reference element and the catalytically active element (i.e., the sensing element) can be larger than signals associated with typical hydrocarbon concentration levels present after the TWC. Although the zero-offset can be reduced by accurate trimming operations, any small resistance drift induced by ageing compromises the long term accuracy of the sensor. Therefore, there is a need in the art to improve the accuracy and the detection limit of calorimetric gas sensors for measuring exhaust gas constituents.